1. Field of the Invention
The present invention relates generally to a semiconductor device including a gate protection circuit, and more specifically to a surge voltage protection circuit provided for a semiconductor device. This protection circuit is necessary for protecting the gate from high-frequency high surge voltage inputted through an input terminal such as a bonding pad and for preventing the semiconductor circuits from being activated erroneously by the surge voltage.
2. Description of the Prior Art
The background of the present invention will be explained with its application to the semiconductor integrated circuit, in particular, to an insulated gate field effect transistor used for an automotive vehicle.
An insulated gate field effect transistor is termed MOS FET since the gate is formed in combination with metal (gate electrode), oxide (gate insulator) and semiconductor, being widely used in various fields. However, since the gate oxide film in this insulated gate field effect transistor is relatively thin and the gate impedance thereof is fairly high, if high-frequency high surge voltage is inputted through the bonding pad from an external device, there are some problems in that the gate is broken down or the circuit formed on a semiconductor silicon substrate is erroneously activated due to the high surge voltage. In applying MOS FETs to an automotive vehicle, in particular, the above-mentioned problems are serious because an automotive vehicle is usually provided with various elements or apparatus which will readily generate high-frequency high surge voltage, for instance, such as an alternator, a starter motor, an ignition system, switches, etc. In more detail, when such an inductive element as described above is turned on or off at a high speed, a kind of electromagnetic wave is generated as electric noise from the inductive element. Therefore, when this electromagnetic noise wave is received by a semiconductor device having a high input impedance such as a field effect transistor, a surge voltage as high as 400 to 500 volts sometimes develops across the input terminals of a semiconductor device.
In order to protect the semiconductor device from the surge voltage, there is well known a method of providing a gate protection circuit, e.g., a surge voltage protection resistor connected between the input terminal and the gate of the field effect transistor. When such a gate protection resistor is additionally provided, since the surge voltage inputted through the bonding pad can be attenuated ohmicly, it is possible to prevent the transistor gate from being broken down or the transistor from being activated erroneously.
By the way, in an automotive vehicle, there often exists a case where the surge voltage develops at the input terminal of a field effect transistor, the voltage of which is as high as several hundred volts and the frequency of which is as high as several magahertz to several tens of megahertz. In the case where such high-frequency high surge voltage is inputted to the device, since the gate protection circuit is formed together with the transistor, it is possible to fairly reduce the surge voltage inputted through the bonding pad 9 before the surge voltage reaches the gate of the transistor, as far as the frequency of surge is relatively low.
However, when the frequency of surge is extraordinary high, the surge current easily leaks from the bonding pad to other circuits formed on the semiconductor substrate, via the substrate, because a distributed capacitance will exist between the bonding pad and the silicon substrate with the insulating silicon oxide film sandwiched therebetween as a dielectric. When surge current leaks from the bonding pad to other circuits formed on the substrate, there exists a danger that the circuits are activated erroneously.
The structure of the prior-art gate protection circuit formed in a semiconductor device will be described in more detail hereinafter with reference to the attached drawings under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.